Transpiler handles `approximation_degree=0.0` as `approximation_degree=None`

[kdk]

As noted in #8595 and elsewhere, the transpiler currently mishandles approximation_degree=0.0 as approximation_degree=None (which is approximation_degree=1.0). Following the definition of approximation_degree here: https://github.com/Qiskit/qiskit-terra/blob/367ed4e34a3d662ef8ed8f2b91e8823bba1b8ca8/qiskit/compiler/transpiler.py#L178

approximation_degree=0.0 should maximally approximate (however that is defined by the relevant pass). Currently, I think there's a bug in handling approximation_degree IIRC in the TwoQubitBasisComposer (by this point, basis_fidelity)

https://github.com/Qiskit/qiskit-terra/blob/dbc81a8dcdae9e9294b9058babf5883258daaa5d/qiskit/quantum_info/synthesis/two_qubit_decompose.py#L1087

that causes this, but this would need to be confirmed first.

[BramDo]

expected_fidelities gives a deviation in the case of approximation_degree=0.0, it gives a exp_fidelity=1 instead of 0 (with basis_fidelity=1). Why is basis_fidelity=1 when approximation_degree=0.0?

[L1092] https://github.com/Qiskit/qiskit-terra/blob/dbc81a8dcdae9e9294b9058babf5883258daaa5d/qiskit/quantum_info/synthesis/two_qubit_decompose.py#L1092)

The reason that basis_fidelity=1 could be that in class TwoQubitBasisDecomposer: basis_fidelity is default 1.

Args:
        gate (Gate): Two-qubit gate to be used in the KAK decomposition.
        basis_fidelity (float): Fidelity to be assumed for applications of KAK Gate. Default 1.0.

The values are 
approx          expr_fid
0       1.0
0.1     0.0010000000000000002
0.2     0.008000000000000002
0.3     0.026999999999999996
0.4     0.06400000000000002
0.5     0.125
0.6     0.21599999999999997
0.7     0.3429999999999999
0.8     0.5120000000000001
0.9     0.7290000000000001
1       1.0

Test code

ap_list = [0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0]
for ap in ap_list:
    circ_10 = transpile(
            circuit,
            basis_gates=["u", "cx"],
            translation_method="synthesis",
            approximation_degree = ap,
        )